SEX IN PROTOZOA 221 



tour nuclei troin the svncaryon in each exconjugant. Two of these 

 become macronuclear anlagen while the other two become functional 

 niicronuclei. Only one cytoplasmic division, accompanied by micro- 

 nuclear division, is required to segregate the two new macronuclei into 

 two daughter cells. Resolution of the old macronuclei into a skein 

 begins about the time of pronuclear formation. The skein later breaks 

 up into fragments \\'hich may persist to some extent until after estab- 

 Hshment of the new nuclear condition in the exconjugants. 



Fiimviechnn viulthincromicleatinn usually has four micronuclei 

 of the same type as those of P. aurelia. At the beginning of conjuga- 

 tion, according to Landis (1925), all four micronuclei go through 

 the first division following crescent formation. All eight of the 

 daughter nuclei undergo the second pregametic division. Of the six- 

 teen nuclei so produced, usually four embark on the third division 

 while the other twelve degenerate. Only one nucleus completes the 

 third division to form the two pronuclei. Again, the location in the 

 cell seems to determine which nuclei will become functional. After 

 cross fertihzation, the conjugants separate and three divisions provide 

 eight nuclei from the syncaryon. Of these, most, probably seven, 

 degenerate. Further divisions provide two macronuclear anlagen and 

 two micronuclei. Four macronuclei are eventually produced, appar- 

 ently by division of the two first formed, while four micronuclei are 

 evident at the same time. By two successive cell divisions, accom- 

 panied by micronuclear divisions, the four new macronuclei are 

 segregated into four cells, each with one macronucleus and four 

 micronuclei. Miiller (1932) found that, although the conjugations as 

 described by Landis occurred in over 60 per cent of the animals, 

 there was a very wide range of variation, especially in the exconju- 

 gants, in which macronuclear anlagen varied from two to fourteen, 

 and micronuclei ranged up to twenty-eight. According to Koster 

 (1933) the fragments of the old macronucleus are distributed among 

 daughter cells during the six to eight divisions following conjugation 

 and are eventually taken bodily into the new macronuclei. 



Presumably all species of Parmnechnn will undergo conjugation 

 under appropriate conditions, but no extended accounts have been 

 published for P. calkinsi, P. ivoodruffi, or P. polycaryum. When P. 

 calk'msi was first described and named by Woodruff (1921), he had 

 not been able to induce conjugation or any other type of nuclear 

 reorganization. Wenrich and Wang (1928) first reported conjugation 

 for this species but gave no details. Recently Wichterman (1948b) 



